I'm a geochemist. In the past ten years I've fixed mass spectrometers, blasted sapphires with a laser beam, explored for uranium in a nature reserve, and measured growth patterns in fish ears, and helped design the next generation of the world's most advanced ion probe. My main interest is in-situ mass spectrometry, but I have a soft spot in my heart for thermodynamics, drillers, and cosmochemistry.

Thursday, December 05, 2013

A recent cartoon/poster on xkcd tries to estimate what the population of habitable zone planets in our stellar neighborhood looks
like. Unfortunately, despite labeling the poster as “all habitable zone
planets”, there are a couple of very important omissions. The center of the picture should look like
this:

When discussing the habitable zone, and how it applies to
exoplanets, one needs to remember that the definition of habitable zone is
sufficiently wide that it covers both Mars and Venus, the closest planets to
Earth. In fact, despite discovering
thousands of exoplanets and exoplanet candidates, we still do not have any
planets as earthlike as Venus. It is hard to say much about exo-Mars
equivalents, as exoplanet detection technology has trouble finding a planet
that small and far from its host star.

Most of the planets shown in the chart have not been
discovered yet. Even among those which
have, very little data about the planets is available. It will be years, perhaps even decades,
before we have the technology to pick an exo-Earth from an exo-Venus. But
framing the exo-planet debate as an Earth versus Venus relative distribution
would be a mistake. Chances are, the
vast majority of these planets are completely unlike either planet.

Our solar system is strange.
It is missing the most abundant type of planet in our galaxy- those
which are larger than Earth, but smaller than Uranus. These worlds are often, albeit deceptively,
referred to as “super-earths”. But as
Systemic has shown, those which we have data for are not only completely
different to anything in our solar system, they are often quite different from
each other.

The omission of Venus and Mars is therefore important,
because it gives the false impression that planets in the habitable zone are
going to be Earthlike. Neither of the habitable zone planets in our solar system are particularly Earthlike, and everything we know about exoplanets so
far suggests that they will be far stranger still.

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